Ronnie O'Sullivan maintained his bid for a record eighth title in the modern era as he opened up a commanding 6-2 lead over Si Jiahui in the first session of their World Championship quarter-final.

Should the Englishman advance on Wednesday, Zhao Xintong looks likely to be his opponent in the last four.

The 28-year-old from China was in dominant mood as he extended his 6-2 advantage to 12-4 over Chris Wakelin in their best-of-25 encounter.

Meanwhile, a compelling contest between four-time champion John Higgins and three-time winner Mark Williams is delicately poised at 8-8 heading into Wednesday's concluding session.

O'Sullivan, who is appearing in only his third competitive match since January, took a scrappy opener before Si responded with a wonderful century break.

However, the 22-year-old from China uncharacteristically missed a series of pots to allow 'The Rocket' to pull clear at 4-1 with three half-centuries.

While Si responded with a 60 of his own to briefly reduce his arrears, another miss, on a black on its spot, proved costly as O'Sullivan pinched the seventh frame.

The Englishman concluded with a stylish 121 break to underline his superiority.

In the remaining quarter-final, Judd Trump produced a superb display of potting as he compiled his 101st and 102nd century breaks of the season to go 5-3 up on Luca Brecel.

Their best-of-25 encounter will continue on Wednesday at 10:00 BST, when Higgins and Williams also return to complete their contest.

World number one Trump is now only one away from equalling the record of 103 century breaks in a single campaign, set by Neil Robertson in 2013-14.

Trump had raced 5-1 ahead, however, Belgium's Brecel, who won in Sheffield two years ago, fought back with breaks of 68 and 128 to leave the match delicately poised.

Scotland's Higgins and Williams are part of snooker's fabled 'Class of 92', which also includes O'Sullivan.

Williams, 50, became the oldest man to reach the last eight at the famous Sheffield venue since Steve Davis did so at the age of 52 in 2010, while Higgins turns 50 on 18 May, 13 days after the Crucible final.

Yet, despite their advancing years, they remain among the game's elite set of players, with this affair reminiscent of the classic they served up in the 2018 final when the Welshman eventually triumphed 18-16.

Having watched Higgins establish a 5-1 lead off the back of four half-century breaks, Williams responded by winning six of the next seven frames, with a run of 104 the highlight in the seventh frame.

World number three Higgins dug in to win the 14th frame and appeared to have recaptured his rhythm with a 70 to edge back in front at 8-7.

However, when he missed a long red to the top right corner in the 16th frame, Williams was able to make a contribution of 65 to level the match.

Zhao's form and rampant display against Wakelin suggest he could well become the first amateur to taste success at the Crucible.

While Wakelin has already defeated 2010 winner Neil Robertson and former world number one Mark Allen, the Englishman found himself totally outplayed in their first two sessions on Tuesday.

While Wakelin, 33, made a break of 93 to take the opening frame in the morning, Zhao then reeled off six in a row.

And the pattern continued when play resumed in the evening, with the 28-year-old racing towards a first semi-final appearance.

Zhao, a former UK Championship winner, was one of 10 players from China to receive a ban in a match-fixing scandal.

He did not directly throw a match, but accepted charges of being a party to another player fixing two matches and betting on matches himself. Zhao - who served a 20-month ban for those offences - is the only of the 10 players sanctioned in the scandal to have returned to action.

As a result of the ban, he lost his place on the World Snooker Tour, although he has already ensured he will be back on the tour full-time next season.

For this tournament, he is officially classed as an amateur and this is his seventh match of the competition, after he had to fight through four qualifying rounds to reach the main draw.